Draco, correct, no evaporator freeze-up (thermostat working very well), and thanks! It is amazing to see the water pouring out of the evaporator box when the system is on full tilt boogie - looks like my car is a water-pumper conversion with a leaking radiator hose just behind the steering rack!

I've taken great pains to block all outside air entry into the cabin (including making a blocking plate for the fresh air intake beneath the windshield - totally non-visible, too), but one of my next projects will be to try out that clear, windscreen film that supposedly does a great job of blocking sun/heat without reducing visibility.